Guanabo is a small seaside town in Havana Province, just east of Playas De Este and Santa Maria del Mar, around 30-40 km east of Havana. The beach apparently is not the nicest, however the town is relatively typically Cuban, there are many casas particulares and some nice restaurants.
Take Bus 400 from Havana for 0.40 CUP. It stops near the train station. Alternatively you can take a taxi for 20 CUC
There are many casa particular. Just ask around!
If you are not into the hotel scene and want to see the real Cuba, Guanabo is the place to go. There is a bus to Havana, and also a train (2km south of town) in season. This is the electric 'Hershey Train', worth seeing for train buffs.
For the hotel scene, Santa Maria del Mar is 6km west.